1. Field of the Invention
The present invention relates to an optical scanning device.
2. Discussion of the Background
With the advance of colorization and high speed performance in an electrophotographic image forming apparatus for use in a laser printer, a digital copier and a facsimile machine, a tandem type image forming apparatus, which has multiple (typically four) image bearing members, has been widely diffused.
As a color image forming apparatus, there is a system which has only one image bearing member and rotates the image bearing member a number of times corresponding to the number of colors. For example, in the case of one image bearing member and four colors, the image bearing member is required to rotate four times, meaning the productivity thereof is inferior.
On the other hand, in an image forming apparatus adopting a tandem arrangement, the number of the light sources inevitably increases. This leads to an increase in the number of component parts, color displacement caused by differences between wavelengths among multiple light sources and an overall cost increase. In addition, deterioration of a semiconductor laser leads to a breakdown of a writing unit (optical scanning device).
As the number of light sources increases, the probability of breakdown naturally increases and thus the opportunity to recycle decreases. This demerit is especially significant when a surface-emitting laser or an D array is used as a light source.
As an optical scanning device which outputs images at a high speed with a small number of light sources, there is known a system described in published unexamined Japanese patent application No. (hereinafter referred to as JOP) 2005-92129.
In this system, the light beam (hereinafter referred to as beam) from a common light source is separated and the respective separated beams are simultaneously directed onto reflection mirrors located in different stacks in a sub-scanning direction (a direction orthogonal to a main scanning direction) to scan different surfaces.
However, in the system described above, beam power from a common light source is reduced to about a half when separated, with a concomitant decrease in the efficiency of the light source power. Consequently, this system requires at least double the power used in a system using a multiple light source arrangement. The increase in power leads to deterioration of the laser beam source and further causes a breakdown of the writing unit.
The deterioration of the laser beam source invites deterioration of writing performance (i.e., deterioration in reliability) and degrades image quality.